3 


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DOCUMEI 


nts'je— 


.  US.  DEPOSITORY 


Hawaii  Agricultural  Experiment  Station, 


HONOLULU. 


E.  V.   WILCOX,  Special  Agent  in  Charge. 


PRESS   BULLETIN    NO.  30. 


Killing  Weeds  With  Arsenite  of  Soda 


E.  V.  WILCOX, 

SPECIAL    AGENT    IN    CHARGE 


INTRODUCTION. 

Weeds  and  pests  of  insect  or  fnngns  nature^^eAiijdfS 
among  the  important  problems  with  which  the  farmer  has  to 
contend  in  all  agricultural  regions.  In  controlling  these  draw- 
•  backs  to  successful  production,  hand  methods  were  first  used 
but  the  use  of  chemicals  was  early  adopted  in  combating  insect 
and  fungus  disease.  Somewhat  later,  sprays  for  the  destruction 
of  weeds  received  attention  and  chemicals  for  weed  destruction 
have  gradually  assumed  a  greater  importance.  Many  of  the 
facts  which  are  now  known  regarding  the  destruction  of  weeds 
by  means  of  chemicals  were  first  discovered  in  connection  with 
spraying  for  insects  and  fungus  diseases.  In  order  to  destroy 
insects  and  pathogenic  fungi  by  applications  of  chemicals  to  liv- 
ing plants  it  is  necessary  to  use  solutions  or  mixtures  of  a 
strength  sufficient  to  be  fatal  to  the  insects  and  fungi  but  not 
injurious  to  the  plants  which  are  to  be  protected.  In  the  exten- 
sive and  long-continued  spraying  experiments  which  have  been 
carried  on  in  farm  practice,  it  was  soon  found  that  certain  forms 
of  chemicals  used  in  such  operations  were  more  injurious  to 


plants  than  other  forms.     Attention,  therefore,  was  required  on 
this  point  in  order  to  avoid  injury  to  the  plants. 

In  the  use  of  arsenicals  for  the  destruction  of  insects  it  is 
necessary  to  get  a  finely  divided  condition  of  the  arsenical, 
which  is  at  the  same  time  relatively  insoluble.  The  small  undis- 
solved particles  then  remain  on  the  surface  of  the  leaves  until 
washed  off  or  eaten  by  insects.  Soluble  arsenicals  are  extremely 
injurious  to  vegetation  and,  therefore,  have  to  be  avoided  in 
spraying  useful  plants.  There  is  thus  a  complete  contrast  in  the 
ideas  aimed  at  in  spraying  for  insects  and  in  spraying  to  kill 
vegetation.  Arsenical  insecticides  must  be  as  insoluble  as  it  is 
possible  to  obtain  them,  while  for  killing  weeds  the  most  soluble 
'orm  of  arsenic  is  desired. 


CHEMICALS   USED   FOR   WEED   DESTRUCTION. 

In  experiments  previously  carried  on  at  this  Station1  carbon 
^  — *\  bisulphide  was  applied  to  a  number  of  weeds  which  grow  in  the 
i  *$£>\#  garden  and  fields.  This  chemical  was  found  to  be  very  effective 
in  the  destruction  of  some  of  our  hardiest  weeds,  both  of  herba- 
ceous  and  shrubby  nature.  When  applied  on  a  large  scale,  how- 
ever, and  in  long-continued  operations,  there  is  some  danger 
from  the  injurious  effects  of  the  fumes  of  carbon  bisulphide  upon* 
the  workmen  and  the  chemical  itself  is  rather  expensive.  It 
seemed,  therefore,  desirable  to  carry  on  experiments  with  other 
chemicals  which  could  be  applied  more  easily  and  at  less  ex- 
pense. Arsenite  of  soda  has  been  extensively  used  on  the  main- 
land of  the  United  States,  in  Australia  and  elsewhere  for  the 
destruction  of  a  variety  of  weed  plants.  Stone2  found  arsenite 
of  soda  exceedingly  effective  against  herbaceous  weeds  when  used 
at  the  rate  of  one  part  in  66  parts  of  water.  Gillette3  in  using 
arsenicals  against  insects  found  that  the  soluble  portion  of  the 
insecticides  exercised  a  much  greater  burning  effect  upon  the 
foliage  in  the  presence  of  dew  and  direct  sunlight.     Kilgore4 


i  Haw.  Sta.  Press  Bui.  No.  25. 

2  Mass.  Rept.  1908,  pt.  I,  pp.  62-72. 

3  Iowa  Sta.  Bui.  10,  p.  419. 

4  N.  C.  Sta.  Bui.  77b. 


observed  that  arsenites  are  more  soluble  in  solutions  containing 
sulphate  of  copper  and  sulphate  of  iron  than  in  water  alone. 
Jones1  used  arsenite  of  soda  on  herbaceous  weeds  and  found  the 
effect  produced  by  this  chemical  to  be  slow  but  lasting.  The 
formula  used  called  for  one  pound  sal  soda  and  two  pounds  of 
water  diluted  before  using  so  as  to  make  nine  gallons.  In  Aus- 
tralia the  common  practice  is  to  buy  arsenite  of  soda  already  pre- 
pared and  simply  dilute  in  water  at  the  rate  of  one  pound  to  five 
to  ten  gallons  of  water.  This  solution  has  proven  effective  in  the 
destruction  of  the  prickly-pear,  wild  blackberry  and  various  her- 
baceous and  semi-shrubby  weeds. 

Asenate  of  soda  has  in  experiments  proven  even  more  effect- 
ive than  the  arsenite.  For  example,  Stone  in  the  article  cited 
above  found  that  the  mixture  of  arsenate  of  soda  and  corrosive 
sublimate  in  equal  parts  caused  a  very  rapid  destruction  of  the 
foliage  and  stems  of  weeds.  Likewise,  Morgan2  found  that  arse- 
nate of  soda  alone  caused  a  rapid  and  destructive  burning  on 
foliage  upon  which  this  chemical  was  applied,  and  Jones  in  the 
article  referred  to  above  reports  arsenate  of  soda  when  used  at 
the  rate  of  one  pound  to  eight  gallons  of  water,  to  be  rather 
quicker  and  more  effective  in  its  action  than  arsenite  of  soda. 

Iron  sulphate.  In  various  processes  involved  in  the  commer- 
cial handling  of  iron  ores,  iron  sulphate  is  obtained  as  a  by- 
product and  costs  about  one  cent  per  pound  in  most  cities  on  the 
mainland.  This  chemical  has  been  widely  tested  in  Wisconsin, 
Iowa,  Maine,  South  Dakota,  Rhode  Island  and  other  states,  as 
well  as  in  foreign  countries,  and  upon  a  large  variety  of  weeds, 
but  chiefly  upon  wild  mustard.  For  this  purpose  it  has  been 
found  particularly  effective  and  has  the  further  advantage  of  not 
destroying  cereal  grains.  For  this  reason  it  is  commonly  sprayed 
over  grain  fields  infested  with  wild  mustard,  and  when  thus  used 
gives  excellent  results.  Iron  sulphate  has  been  used  in  different 
strengths,  but  where  the  herbage  is  quite  tender  a  20  per  cent 
solution  is  the  one  commonly  recommended  to  kill  weeds  and 
not  injure  cereals.     Iron  sulphate  is  readily  soluble  in  water, 


i  Vt.  Sta.  Rpt.  1901,  p.  248. 

2  Agric.  Education,  6  (1903)  p.  11. 


and  no  other  manipulation  is  required  for  the  spray  than  simply 
to  dissolve  the  chemical  in  water.  At  the  Station  iron  sulphate 
has  been  used  upon  spurge,  cockle-bur,  lantana,  oi,  pig-weed, 
Japanese  nut-grass  and  other  weeds.  When  used  at  the  rate  of 
three  pounds  per  gallon  of  water,  iron  sulphate  killed  all  these 
weeds  except  lantana  and  Japanese  nut-grass.  The  leaves  were 
destroyed  on  lantana  but  the  new  buds  were  sent  out,  showing 
that  the  stems  were  not  killed.  The  portion  of  Japanese  nut- 
grass  above  ground  soon  turned  brown  and  died  after  being 
sprayed  with  iron  sulphate,  but  fresh  shoots  came  up  from  the 
under-ground  bulbs.  This  chemical  can  be  used  effectively 
against  most  weeds  except  lantana  and  Japanese  nut-grass,  and 
has  the  advantage  of  not  being  poisonous  or  in  any  way  injur- 
ious to  the  soil. 

Carbolic  Acid.  When  diluted  at  the  rate  of  1  quart  in  8 
gallons  of  water,  Jones1  observed  little  effect  on  common  weeds 
from  spraying  with  carbolic  acid.  Qn  the  other  hand,  Voelcker2 
obtained  satisfactory  results  from  spraying  wild  onion  with  a 
5  per  cent  solution  of  carbolic  acid,  and  the  same  chemical  proved 
effective  in  experiments  on  miscellaneous  weeds  at  the  Massa- 
chusetts Experiment  Station. 

Benzine.  This  substance  has  been  tested  on  a  small  scale  in 
destroying  weeds  in  gardens  in  various  localities.  Stone3  reports 
that  a  50  per  cent  mixture  of  equal  parts  benzine  and  gasoline 
in  water  was  effective  against  the  less  hardy  weeds. 

Kerosene.  In  preparing  kerosene  mixture  as  a  spray  for 
sucking  insects,  it  is  always  recommended  that  care  be  taken  not 
to  leave  any  of  the  kerosene  uncombined  with  the  soap  in  the 
mixture,  for  the  reason  that  if  uncombined  the  kerosene  may 
burn  the  foliage.  Kerosene  has  a  slight  burning  effect  on  the 
foliage  of  various  trees.  It  was  tried  by  Wilson4  as  a  spray  for 
weeds  at  the  rate  of  6%  gallons  per  square  rod.     When  thus 


i  Vt.  Sta.  Rept.  1901,  p.  248. 

2  Jour.  R.  Agr.  Soc.  Eng.  63  (1902)   p.  360. 

3  Mass.  Rept.  1908,  Pt.  I,  pp.  62-72. 

4  Minn.  Sta.  Bui.  95. 


used  it  was  not  efficient  enough  to  warrant  its  application  on  a 
commercial  scale. 

Salt.  The  injurious  effect  of  salt  upon  vegetation  is  a  mat- 
ter of  common  knowledge.  Plants  show  some  variation  in  their 
resisting  power  to  salt,  but  a  maximum  salt  content  is  soon 
reached  in  the  moisture  of  the  soil,  after  which  the  plants  die  or 
can  not  thrive.  Salt  has  been  applied  for  killing  weeds,  either 
dissolved  in  water  or  sprinkled  upon  the  ground  about  the  weeds 
to  be  destroyed.  Salt  is  chiefly  valuable  for  this  purpose  in  side- 
walks or  roadsides  where  it  is  not  desired  to  grow  cultivated  crops 
after  the  weeds  are  destroyed.  In  such  places  there  is  no  harm 
from  the  presence  of  large  quantities  of  salt  in  the  soil.  As  a 
spray  it  has  been  found  that  salt  applied  at  the  rate  of  80  bar- 
rels is  inefficient  for  the  destruction  of  field  weeds.  It  is,  there- 
fore, scarcely  to  be  recommended  in  the  list  of  commercial  weed 
destroyers  in  field  practice. 

Sulphuric  acid.  On  account  of  its  great  oxidizing  and  burn- 
ing power,  sulphuric  acid  has  frequently  been  thought  of  as  a 
weed  destroyer.  AVhen  used  in  a  15  per  cent  solution  in  water, 
Wilson1  found  sulphuric  acid  to  be  very  efficient  for  destroying 
weeds  but  rather  too  expensive.  Similar  results  have  been  ob- 
tained in  Maine  and  elsewhere.  Sulphuric  acid  is  objectionable 
on  account  of  the  danger  connected  with  its  use  by  ordinary  la- 
borers, and  its  destructive  effect  on  spraying  apparatus.  At  the 
Station  sulphuric  acid  was  tried  as  a  means  of  destroying  guava, 
but  without  satisfactory  results. 

Copper  sulphate.  Copper,  both  in  the  form  of  sulphate  and 
nitrate,  but  particularly  the  former,  has  been  widely  used  in 
killing  various  kinds  of  weeds,  especially  wild  mustard.  The 
effectiveness  of  copper  sulphate  for  this  purpose  has  been  thor- 
oughly demonstrated,  both  in  Europe  and  the  United  States. 
A  3  to  5  per  cent  solution  is  commonly  recommended  to  be 
applied  as  a  spray.  Hitier2  had  excellent  results  in  destroying 
wild  mustard  from  spraying  with  a  4  per  cent  solution  of  copper 


i  Minn.  Sta.  Bui.  95. 

2  Jour.  Agr.  Prat.  5   (1905)   pp.  65-8. 


6 


sulphate  or  a  3  per  cent  solution  of  copper  nitrate.  Copper 
sulphate  may  he  added  with  a  slight  advantage  to  a  solution  of 
arsenite  of  soda,  since,  when  thus  used,  it  increases  the  solubil- 
ity and  burning  effect  of  the  arsenite. 

Ammonia.  Ammonia  has  also  been  used  as  a  spray  for  the 
destruction  of  weeds  but  cannot  be  recommended  for  general  use 
on  account  of  its  slight  burning  power  and  the  disagreeable 
fumes  which  annoy  the  laborers. 

Arsenite  of  soda.  This  chemical  is  usually  prepared  by  boil- 
ing together  common  white  arsenic  and  washing  soda  or  sal  soda. 
Theoretically  these  chemicals  combine  in  such  a  proportion  that 
if  about  1%  pounds  of  soda  be  used  for  .each  pound  of  white 
arsenic,  the  proper  combination  will  take  place  to  render  all  of 
the  arsenic  soluble  by  the  formation  of  arsenite  of  soda.  In 
practice  a  considerably  larger  proportion  of  soda  has  been  used, 
up  to  4  pounds  per  pound  of  white  arsenic.  It  seems  unneces- 
sary, however,  to  use  more  than  2  pounds  of  soda  per  pound  of 
white  arsenic.  In  preparing  arsenite  of  soda  we  have  had  good 
results  from  boiling  a  mixture  of  one  pound  white  arsenic  and 
two  pounds  sal  soda  per  gallon  of  water  for  from  15  to  20  min- 
utes. The  exact  length  of  time  to  continue  the  boiling  can  not 
be  stated  beforehand  with  any  certainty  but  the  boiling  should 
be  continued  until  the  solution  becomes  clear.  The  clearing  of 
the  solution  indicates  that  the  proper  chemical  combination  has 
taken  place.  In  Australia,  where  arsenite  of  soda  has  been 
widely  used  for  killing  weeds,  the  common  practice  is  to  buy  the 
arsenite  of  soda  as  such  on  the  market,  but  in.  the  spraying  thus 
far  done  in  Hawaii  with  arsenite  of  soda,  it  has  been  prepared 
by  boiling  white  arsenic  and  sal  soda  as  just  described.  The 
stock  solution  obtained  by  boiling  together  the  sal  soda  and  white 
arsenic  is  to  be  diluted  before  spraying  with  15  to  24  parts  of 
water,  depending  on  the  hardiness  of  the  weeds  to  be  destroyed. 

Arsenate  of  soda.  This  chemical  is  about  equally  soluble 
with  arsenite  and  where  obtainable  on  the  market  at  a  reason- 
able price  may  be  substituted  for  the  latter.  In  experiments 
with  miscellaneous  weeds,  Jones1  found  arsenate  of  soda  very 


effective  when  used  at  the  rate  of  1  pound  to  8  gallons  of  water. 
Similarly  in  Massachusetts  arsenate  of  soda  gave  good  results 
as  a  weed  destroyer,  either  when  used  alone  or  when  mixed  with 
corrosive  sublimate  in  equal  parts. 

EXPERIMENTS   WITH   ARSENITE   OF   SODA. 

Before  giving  an  account  of  the  experiments  with  arsenite  of 
soda  at  this  Station  and  elsewhere  in  Hawaii,  it  may  be  well  to 
refer  briefly  to  other  experiments  with  the  same  chemical.  In 
Australia  this  spray  has  been  widely  used  for  the  destruction  of 
prickly-pear,  wild  blackberry  and  other  shrubby  and  herbaceous 
weeds.  For  this  purpose  it  has  proven  so  effective  as  to  make  a 
wide  field  for  itself  in  the  programme  of  weed  destruction. 
Maiden,2  on  the  basis  of  a  number  of  experiments,  recommends 
for  destroying  prickly-pear  a  solution  of  arsenite  of  soda  con- 
taining 1  pound  to  from  5  to  9  gallons  of  water.  In  other  exper- 
iments more  concentrated  solutions  were  used,  particularly  for 
the  destruction  of  wild  blackberry  and  other  shrubby  weeds. 
Likewise  in  Iowa,  Xorth  Carolina,  Vermont,  Massachusetts  and 
elsewhere  on  the  mainland,  as  well  as  in  Europe,  arsenite  of 
soda  has  been  used  with  good  success  in  destroying  miscellaneous 
herbaceous  weeds.  In  Massachusetts  it  proved  effective  when 
used  at  the  rate  of  1  part  to  66  parts  of  water.  Kilgore3  found 
that  when  an  attempt  was  made  to  combine  arsenical  insecticides 
containing  soluble  arsenic  with  fungicides  containing  copperas, 
copper  sulphate  or  iron  chloride,  the  burning  effect  of  the  arse- 
nite was  considerably  increased  by  the  presence  of  the  other  min- 
eral salts.  In  spraying  experiments  for  the  destruction  of  insects 
Gillette4  observed  that  soluble  arsenites  in  arsenical  sprays  pro- 
duced a  greater  burning  effect  in  the  presence  of  dew  and  direct 
sunlight.  These  points  are  worth  considering  as  indicating  the 
conditions  under  which  applications  of  arsenite  of  soda  should 
be  made  in  order  to  obtain  a  maximum  effect. 


i  Vt.  Sta.  Rept.  1901,  p.  248. 

2Agr.  Gaz.  X.  S.  W.  9  (1898),  p.  984. 

3  X.  C.  Bui.  77b. 

4  Iowa  Sta.  Bui.  10,  p.  419. 


8 


At  this  Station  experiments  have  been  carried  on  with  arse- 
nite  of  soda  in  testing  the  effect  of  this  chemical  upon  a  number 
of  weeds,  including  oi  (Stachytarpketa  dichotoma) ,  lantana, 
spurge  (Euphorbia  peplus),  pualele  or  sow  thistle,  pig-weed, 
purslane,  cockle-bur  (Xanthium  strumarium) ,  glue  (Acacia  far- 
nesian-a) ,  dodder,  Japanese  nut-grass,  honohono  (Commelina 
nudiflora),  crotalaria  and  other  weeds.  In  these  experiments 
the  stock  solution,  as  described  above,  was  dissolved  in  from  15 
to  20  parts  of  water.  The  effect  of  the  spray  was  manifest  in 
most  instances  within  2  or  3  hours,  but  on  Japanese  nut-grass 
the  leaves  did  not  turn  brown  until  the  second  day.  On  all  of 
these  weeds  the  leaves  and  stems  ultimately  died  as  a  result  of 
a  single  application. 

In  spraying  alfalfa  infested  with  dodder  the  alfalfa  was 
killed  as  well  as  the  dodder.  In  this  respect  the  results  were  the 
same  as  those  obtained  with  spraying  with  iron  sulphate.  The 
leaves  and  small  stems  of  lantana  were  destroyed  by  a  single 
application,  but  for  the  complete  destruction  of  the  plant  a  sec- 
ond, third  or  even  fourth  treatment  was  found  necessary.  With 
sow  thistle,  although  apparently  destroyed,  it  seemed  to  recover 
rather  promptly  and  grew  up  again  from  the  base  of  the  plant. 
Similar  results  have  been  had  in  ^N"ahiku  in  spraying  this  plant. 
Some  difficulty  is  likely  to  be  experienced  in  destroying  hono- 
hono on  account  of  the  fact  that  in  dense  patches  it  is  hard  to 
reach  all  portions  of  the  plant  with  the  spray  and  two  or  three 
applications  may  be  necessary.  Japanese  nut-grass,  although 
entirely  destroyed  above  ground  by  the  arsenite  of  soda  spray, 
promptly  grew  up  again  from  the  underground  bulbs  without 
showing  any  serious  diminution  of  vigor.  In  previous  experi- 
ments of  the  Station  it  was  found  possible  to  destroy  this  plant 
with  carbon  bisulphide,  but  this  chemical  is  too  expensive  for 
use  on  a  large  scale.  Judging  from  the  experiments  which  have 
thus  far  been  made,  the  best  way  to  destroy  this  weed  economi- 
cally is  to  shade  it  out  by  using  pigeon-peas  and  allowing  them 
to  occupy  the  ground  for  a  year  or  more. 

In  the  practical  application  of  arsenite  of  soda  on  a  large 


scale  in  Hawaii,  the  chief  credit  is  due  to  Mr.  W.  A.  Anderson, 
Manager  of  the  Nahiku  Rubber  Company.  In  consequence  of 
the  satisfactory  results  which  he  has  obtained,  the  method  has 
been  generally  adopted  by  all  of  the  other  rubber  companies. 
More  than  500  acres  of  land  covered  with  miscellaneous  shrubby 
and  herbaceous  weeds  have  already  been  cleaned  by  means  of  the 
spray,  and  ultimately  the  whole  area  now  planted  to  rubber  will 
be  treated  in  this  way.  Some  of  the  experience  of  Mr.  Anderson 
is  stated  as  follows  in  a  letter  from  him: 

"We  use  it  for  all  troublesome  grasses  according  to  the  following 
formula:  2  pounds  crystal  sal  soda,  1  pound  arsenic,  boiled  in  1  gal. 
water  until  clear.  This  is  diluted  in  about  24  gals,  of  water  in  the 
field  and  applied  at  the  rate,  roughly,  of  about  100  gals,  of  the  diluted 
mixture  per  acre,  more  or  less  according  to  the  length  and  thickness  of 
the  growth. 

"We  find  this  effective  for  all  the  grasses  we  have  to  deal  with,  and 
for  the  Hitchcock  berry  and  for  the  general  run  of  weeds,  excepting 
pualele  (milkweed),  ginger,  and  wild  taro.  For  best  results  on  hono- 
hono  a  little  stronger  solution,  perhaps  one  gallon  of  stock  mixture  to 
20  of  water,  is  necessary.  Diluted  1  to  24,  it  is  partially  effective  with 
the  three  exceptions  given  above,  and  I  feel  sure  that  used  slightly 
stronger,  it  would  dry  them  up,  as  it  does  the  grasses  and  ordinary 
weeds. 

"In  a  complete  list  of  the  plants  affected,  lantana  could  not  be 
omitted,  as  it  burns  the  leaves  off,  and  I  have  in  mind  a  patch  treated 
over  a  year  ago  which  has  not  grown  yet.  It  is  unquestionably  safe 
to  say  lantana  can  be  kept  in  check  with  it,  the  fact  having  been  dem- 
onstrated that  the  growth  above  ground  can  be  killed,  the  conclusion 
would  be  natural  that  by  keeping  at  it,  the  roots  in  time  might  be 
starved  out. 

"Hilo  grass  is  actually  disappearing  from  our  fields  that  have  been 
plowed,  and  thereafter  treated  with  the  spray.  Considerable  areas  are 
entirely  free  from  it,  and  it  has  been  thinned  out  everywhere." 

For  several  years  the  Hitchcock  berry  or  thimble  berry,  and 
German  ivy  (Senecio  mihanioides)  have  been  encroaching  on  the 
grass  lands  of  some  of  the  ranches  on  Hawaii,  particularly  the 
Parker  Ranch.  The  method  of  eradication  at  first  adopted  was 
that  of  digging  out  by  hand.  In  this  way  about  $1500  per 
month  was  being  expended  on  these  weeds  and  the  financial  bur- 
den seemed  an  excessively  heavy  one  for  the  single  item  of  weed 
eradication.  As  soon  as  the  experiments  with  arsenite  of  soda 
had  been  carried  far  enough  to  warrant  a  specific  recommenda- 


10 


tion  it  was  urged  that  this  spray  be  adopted  for  the  weeds  on 
ranch  land.  An  extensive  test  was  at  once  begun  with  results 
which  have  justified  the  general  application  of  arsenite  of  soda. 
!Not  only  the  Hitchcock  berry  and  German  ivy  are  destroyed  by 
this  spray  but  also  the  nettle  (Hesperocnide  sandwichensis) . 
These  are  the  very  worst  weeds  on  ranch  land.  The  use  of  arse- 
nite of  soda  as  a  spray  seems  to  be  a  practical  solution  of  the 
problem  of  their  eradication.  Large  bushes  of  Hitchcock  berry 
are  destroyed  by  a  single  application.  At  any  rate  the  roots 
have  been  found  to  be  dead  for  a  distance  of  18  inches  from  the 
base  of  the  trunk  and,  of  course,  the  leaves  and  the  stems  were 
all  killed  by  the  same  treatment. 

The  method  of  application  of  arsenite  of  soda  on  rough  ranch 
land  or  in  rubber  plantations  is  a  simple  one.  The  most  of  these 
lands  do  not  admit  of  horse-drawn  spraying  apparatus  on  account 
of  the  junglelike  nature  of  the  weeds  and  the  roughness  of  the 
lands.  A  knapsack  spray  holding  about  5  gallons  of  solution 
may  be  carried  on  the  backs  of  laborers,  who  can  readily  make 
their  way  through  the  bushes  and  grass,  covering  the  vegetation 
with  the  spray  as  they  walk  along.  One  laborer  can  spray  from 
one  to  two  acres  per  day,  depending  upon  the  height  and  density 
of  the  weeds  to  be  sprayed  and  the  character  of  the  land.  The 
total  cost  of  a  single  application,  including  materials  and  labor, 
varies  from  $1.25  to  $2.25  per  acre.  The  flowers  and  tender 
foliage  of  all  weeds  are  destroyed  by  a  single  application  and 
this  effectively  prevents  the  immediate  spread  of  the  weeds.  As 
a  rule  the  stems  and  roots  of  the  weeds  are  destroyed  by  the 
first  application.  Hilo  grass,  however,  lantana  and  certain  other 
of  the  hardy  weeds  may  require  two  or  three  applications  for 
their  complete  destruction.  About  100  gallons  of  the  diluted 
spray  is  usually  sufficient  for  an  acre  of  ground. 

In  spraying  with  arsenite  of  soda  it  should  be  remembered 
that  the  effect  is  produced  by  contact  with  the  aerial  portions  of 
the  plant.  The  spray  should  be  applied  in  the  form  of  a  fine 
mist  so  as  merely  to  cover  the  surface  of  the  leaves  and  the 
stems.     Care  should  be  exercised  not  to  drench  the  plants,  since 


11 


no  more  effect  would  be  thus  obtained,  and  the  risk  would  be  run 
of  introducing  too  much  arsenic  into  the  soil. 

At  Ulupalakua  some  losses  have  been  experienced  in  the 
death  of  cattle  from  eating  a  poisonous  passion  vine  which  has 
gradually  spread  over  a  small  area  of  the  ranch  land.  The  arse- 
nite  of  soda  is  now  being  used  for  destruction  of  this  plant  and 
also  to  kill  the  air  plant  (Bryophyllum  calycinum),  a  useless 
weed  which  covers  some  of  the  range  and  prevents  other  plants 
from  growing.  It  is  commonly  considered  that  red  sage  (Salvia 
coccinea)  causes  abortion  in  cows  and  mares.  The  evidence  for 
this  belief  is  quite  conclusive  and  has  come  from  a  number  of 
ranches.  The  arsenite  of  soda  is  now  being  used  on  Lanai  for 
the  destruction  of  this  weed. 

The  chemical  method  for  the  destruction  of  weeds  as  de- 
scribed in  this  bulletin  is  capable  of  a  much  wider  application, 
particularly  for  killing  weeds  along  roadways  and  in  waste 
places  where  cultivation  is  at  present  out  of  the  question.  It 
should,  of  course,  be  remembered  that  the  arsenite  is  poisonous 
and  stock  animals  should  not  be  allowed  to  browse  upon 
sprayed  vegetation  until  after  sufficient  rain  has  fallen  to  wash 
off  what  may  remain  on  the  foliage.  As  a  rule  the  effect  of  the 
spray  begins  to  be  manifested  within  a  few  hours  in  the  wither- 
ing and  browning  in  the  leaves  of  sprayed  plants.  They  are 
thus  rendered  unpalatable  as  feed  for  animals.  The  spray  must 
remain  in  contact  with  the  foliage  for  at  least  two  hours,  in 
order  to  have  the  desired  effect.  In  applying  arsenite,  therefore, 
a  clear  day  should  be  chosen,  or  at  least  one  in  which  it  is  not 
likely  to  rain  soon  after  the  application  of  the  pray. 

POSSIBLE   DANGER    FROM   THE    USE    OF 
ARSENITE  OF  SODA. 

In  addition  to  the  danger  of  stock  becoming  poisoned  from 
grazing  on  sprayed  vegetation,  it  seems  desirable  to  consider  the 
possible  injurious  effect  of  arsenic  upon  the  soil  and  cultivated 
crops  which  may  be  growing  in  the  soil  at  the  time,  or  which  may 


12 


be  subsequently  planted  in  it.  This  matter  has  already  received 
some  attention,  particularly  by  the  Colorado  Experiment  Sta- 
tion. There  is  still  some  difference  of  opinion  as  to  the  possible 
harmfulness  of  continued  spraying  with  arsenic.  Bishop1  made 
analyses  of  potatoes  after  the  vines  had  been  sprayed  with  Paris 
green  and  found  that  they  contained  .0002  per  cent  of  arsenic. 
Breteau2  made  an  examination  of  wine  obtained  from  grapes 
picked  from  vines  which  had  been  sprayed  with  arsenicals  and 
found  .  003  to  .  2  mg  of  arsenic  per  liter  of  wine.  Comere3  cul- 
tivated algse  in  solutions  of  potasium  arsenate.  This  investiga- 
tor found  that  algae  not  only  grew  well  in  nutritive  solutions  con- 
taining arsenate  but  that  arsenic  acid  could  replace  phosphoric 
acid  in  such  plants.  Hyslop4  found  that  in  cattle  which  had  been 
sprayed  with  arsenical  solutions,  arsenic  was  present  in  urine 
but  not  in  meat  or  milk.  Collins0  made  cultures  with  barley  in 
which  arsenic  was  added  at  the  rate  of  11  pounds  per  acre  nor- 
mally. The  grain  of  barley  grown  in  the  pots  contained  arsenic 
to  the  extent  only  of  4  oz.  per  acre  and  the  straw  1%  oz.  per 
acre.  Arsenic  has  been  found  in  minute  quantities  in  the  fruit 
and  in  all  parts  of  fruit  trees  which  have  been  continuously 
sprayed  with  arsenic.  It  is  well  known  that  plants  readily  ab- 
sorb arsenic  from  the  soil  but  a  considerable  variation  exists  in 
their  sensitiveness  to  this  chemical.  In  the  long-continued  liti- 
gation between  the  smelters  .and  cattlemen  in  Deer  Lodge  Val- 
ley, Montana,  an  excellent  opportunity  was  had  of  studying  the 
effect  of  arsenic  upon  vegetation  and  live  stock.  Unfortunately, 
too  little  attention  was  given  in  this  extensive  investigation  to 
the  effect  of  arsenic  upon  plants  when  absorbed  into  their  tissues. 
The  arsenic  in  this  case  was  deposited  largely  upon  the  surface 
of  the  plants  in  dust  which  settled  from  smelter  fumes  and 
which  contained  arsenic  in  large  quantities.     In  all  cases,  how- 


1  Jour.  Am.  Chem.  Soc.  28  (1906),  p.  184. 

2  Jour.  Pharm.  China.  28   (1908),  p.  156. 

•3  Bui.  Soc.  Bot.  France  56  (1909),  p.  147. 

4  Natal  Agr.  Jour.  15  (1910),  pp.  693-697. 

5  Jour.  Soc.  Chem.  Ind.  21  (1902),  pp.  221-222. 


33 


ever,  in  which  careful  analyses  were  made,  arsenic  was  found 
to  have  been  absorbed  into  the  substance  of  grasses.6 

The  most  extensive  study  of  the  effect  of  the  absorption  of 
arsenic  upon  plants  has  been  carried  out  by  Headden.1  Many 
fruit  trees,  both  apple  and  pear,  were  observed  to  be  in  a  sickly 
condition  or  dying  in  the  orchards  of  Colorado  which  had  re- 
ceived arsenical  sprays  for  from  20  to  40  years.  An  examina- 
tion of  the  soil  under  such  trees  showed  the  presence  of  arsenic 
in  varying  quantities,  the  highest  being  138  parts  per  million. 
In  some  cases  where  sickness  or  death  of  the  trees  was  observed 
the  arsenia  content  of  the  soil  was  at  least  ten  times  that  of  nor- 
mal soils  in  that  locality.  The  orchardists  were  found  to  have 
been  applying  for  years  what  must  be  considered  as  excessive 
amounts  of  arsenic.  In  some  cases  as  much  as  0.9  of  a  pound  per 
tree.  In  a  period  of  six  years  this  would  amount  to  432  pounds 
per  acre  or  about  108  parts  per  million  in  the  first  foot  of  soil. 
The  first  symptom  of  poisoning  in  the  orchard  trees  was  a  pre- 
mature yellowing  of  the  leaves.  If  the  application  of  the  arseni- 
cals  was  repeated  the  next  year  the  trees  died  the  second  year. 
Serious  injuries  were  also  produced  in  these  orchards  from  the 
corrosive  effect  of  the  arsenic  which  ran  clown  the  trunks  of  the 
trees.  The  bad  effects  were  most  noticeable  at  the  collar  of  the 
trunk  near  the  surface  of  the  ground. 

In  view  of  the  somewhat  divergent  opinions  which  have  thus 
far  been  expressed  relative  to  the  effect  of  arsenic  upon  plants, 
we  may  well  inquire  what  may  be  considered  the  outlook  from 
the  continued  use  of  arsenite  of  soda  as  a  weed  destroyer  in 
Hawaii.  The  results  announced  by  Headden  and  referred  to 
above  have  been  called  in  question  by  other  writers  but  without 
substantial  evidence  to  disprove  his  position.  Our  experience  in 
Hawaii  is  of  only  two  years'  duration.  In  a  letter  from  Mr.  W. 
A.  Anderson  of  Xahiku,  Maui,  the  following  statement  is  made 
on  the  point  under  consideration : 

"As  you  know,  we  have  been  using  the  spray  for  nearly  two  years 


6  Jour.  Am.  Chem.  Soe.  30  (1908),  pp.  915-946. 
i  Colo.  Sta.  Buls.  131  and  157. 


14 


now,  in  quantities,  and  have  not  been  able  yet  to  observe  any  injurious 
effects  on  the  trees.  Where  it  has  been  applied  frequently  enough  to 
keep  the  ground  in  a  measure  free  from  weeds,  a  marked  improve- 
ment is  noticed  in  the  appearance  of  the  soil,  as  I  suppose  might  be 
expected  from  exposing  it  to  the  air." 

In  applying  the  arsenite  spray  on  the  rubber  plantation  of 
the  Pacific.  Development  Company  a  considerable  quantity  of 
the  solution  was  accidentally  upset  near  two  rubber  trees  and 
the  leaves  fell  from  these  trees  within  two  days  apparently  from 
the  poisonous  effects  of  the  arsenite.  The  trees,  however,  are 
recovering. 

The  conditions  under  which  arsenite  of  soda  has  been  applied 
in  Hawaii  differ  greatly  from  those  which  prevail  in  apple  or- 
chards in  Colorado.  In  the  first  place  we  are  applying  only 
five  pounds  of  arsenic  per  acre  and  to  soils  which  normally  con- 
tain no  arsenic.  Then,  too,  the  rainfall  in  rubber  plantations  is 
very  high  (160  to  200  inches  per  year).  In  the  spraying  exper- 
iments in  the  Colorado  apple  orchards  the  most  insoluble  form 
of  arsenic  was  used.  When  washed  down  into  the  soil  it,  there- 
fore, remains  for  a  long  time,  gradually  becoming  soluble  and 
being  absorbed  by  the  roots  of  the  trees.  Arsenite  of  soda  is  an 
extremely  soluble  form  of  arsenic.  It  is  not  known  whether  a 
considerable  quantity  of  the  arsenite  of  soda  may  subsequently 
be  fixed  in  the  soil  by  interaction  with  other  chemicals  found  in 
the  soil.  The  soils  in  the  rubber  plantations,  however,  are  ex- 
tremely porous  and  are  underlaid  with  a-a  to  such  an  extent  that 
running  streams  are  rare.  It  is  highly  probable,  therefore,  that 
the  most  of  the  arsenite  of  soda  washed  into  the  soil  by  rains  is 
carried  away  by  the  water  passing  through  the  soil.  It  seems 
very  doubtful  whether  any  serious  accumulation  of  arsenic  can 
take  place  in  the  soils  of  the  rubber  plantations  so  long  as  the 
conditions  remain  as  at  present.  It  can  not  be  questioned,  how- 
ever, that  arsenic  in  excessive  quantities  in  the  soil  is  injurious 
to  nearly  all  forms  of  vegetation,  and,  therefore,  some  care 
should  be  observed  in  not  using  unnecessarily  large  quantities  of 
the  arsenical  spray.  No  harmful  results  have  thus  far  been  ob- 
served upon  rubber  or  other  cultivated  plants  in  Hawaii  from 


15 


arsenicals  and  it  is  not  likely  that  harm  will  result,  at  least  in 
porous  soils  such  as  prevail  in  rubber  plantations,  particularly 
with  the  very  small  quantities  of  arsenic  which  are  being  used. 
The  effects  of  arsenic  will  be  closely  observed  and  if  any  injury 
should  manifest  itself  in  years  to  come,  the  danger  may  be 
avoided  by  adopting  for  a  year  or  so  the  harmless  but  somewhat 
less  effective  method  of  spraying  with  sulphate  of  iron. 


UNIVERSITY  OF  FLORIDA 


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